They are better then the stock manifold but not much. On a stock car they won't do alot but its one of those things that the more you do will become more beneficial.
I bought a set of BBK L/T's with 6,000 miles on them for $75. If you look around, you'll find a deal. I personally wouldn't bother with shorties. I've had them before and they're a waste of money. And not to mention the install in a PITA!!! :rant:
Id shop around for a used pair of L/T's. You wont see much of a gain between the bbk shorties and your stock manifold. The only shorties i've heard of that are worth the swap are JBA's. Otherwise, not worth the hassle.
if you do headers i would go with stainless or ceramic coated. i seen a dyno with the bbk shorty headers when they came out and it picked up 8 hp. it did loose about 4 or 5 down low. :drunk: